The present disclosure relates to surfaces that exhibit superhydrophobic properties when treated with a fluorine-free composition applied with a water-based, non-organic solvent.
A superhydrophobic surface exhibits a sessile water contact angle of greater than 150°. If, additionally, the surface exhibits a water droplet roll-off (sliding) angle of less than 10°, the surface is deemed to be “self-cleaning.” In nature, lotus leaves exhibit such properties (so-called lotus effect). Most man-made materials such as fabrics, nonwovens, cellulose tissues, polymer films, etc., do not have surfaces with such properties. Currently, there are several methods to modify a non-superhydrophobic surface to achieve the lotus effect. One method is to graft hydrophobic polymer(s) (using a monomer, co-monomers, etc.) onto every exposed surface of a non-superhydrophobic material. Such a method makes the material superhydrophobic throughout the thickness of the material, which might not be desired in most cases. It is also not cost effective, cannot be used for a continuous production, and can lead to undesirable environment issues.
The development and implementation of water-based, non-fluorinated formulations for bio-inspired superhydrophobic surface treatments can greatly reduce the adverse environmental impact typically associated with their synthesis. Over the past several decades, many approaches to these superhydrophobic surfaces have been developed that commonly require harsh organic solvents, complex processing methods, and/or environmentally undesirable fluorinated chemistry. In addition, many of the demonstrated methods are not relevant in practice on large scales in commercial application, not only for their negative consequences to the environment, but also the inability to economically prepare large-area fluid repellent surfaces at sufficiently low-cost. Imparting liquid repellency via large-area approaches, such as spray-casting or size press coating, have been shown to be viable for low-cost and substrate-independent fluid management. Previously, a likewise water-based and non-fluorinated superhydrophobic formulation was presented achieving nanometer-scale roughness via exfoliated graphite nanoplatelets (GNP, a.k.a. multilayer graphene); unfortunately, this formulation had an opacity and dark color, limiting its versatility in many commercial applications. More importantly, this approach required pH adjustment to improve suspension stability.
A standard approach is to coat a specially-formulated liquid dispersion onto a surface. Upon subsequent drying, a nano-structured superhydrophobic film forms. To use such an approach, the deposited film must exhibit a chemical and physical morphology characteristic of superhydrophobic surfaces. First, the formulation requires at least one low-surface energy (i.e., hydrophobic) component, and second, the treated surface has to have a rough surface texture, preferably extending over several length-scales characteristic of micro- and/or nano-roughness. Although various formulated dispersions capable of achieving a superhydrophobic surface exist, rarely are they purely water-based and they generally contain harmful fluorinated compounds to reduce surface energy.
Low-cost, large-area superhydrophobic coating treatments are of great value to many applications requiring a passive means for attaining efficient liquid repellency. While many applications are envisioned, only few are realizable due to either the high-cost or low-durability of such treatments. Recently, spray deposition of polymer-particle dispersions has been demonstrated as an excellent means for producing low-cost, large-area, durable, superhydrophobic composite coatings/films; however, the dispersions used for spray deposition of superhydrophobic coatings generally contain harsh or volatile solvents. Solvents are required for wet processing of polymers, as well as for dispersing hydrophobic nanoparticles, thus inhibiting scalability due to the increased cost in chemical handling and safety concerns. This problem can be overcome by replacing solvents with water, but this situation is paradoxical: producing a highly water-repellent coating from an aqueous dispersion.
Also, such coatings usually contain fluoropolymers. A low-surface energy fluoropolymer (e.g., fluoroacrylic copolymers, poly(tetrafluoroethylene), etc.) is typically incorporated into the formulation to achieve liquid repellency. However, concerns over their bio-persistence have provided an impetus for eliminating these chemicals. The problems with the byproducts of fluoropolymer degradation, e.g. long-chain perfluorinated acids (PFAs) that have a documented ability to bioaccumulate, as well as the potential adverse effects PFA in maternal concentrations can have on human offspring, have led to a shift in the manufacture and usage of fluoropolymers. One common PFA of particular concern is perfluorooctanoic acid (PFOA). In 2006, the EPA introduced its PFOA (perfluorooctanoic acid) Stewardship Program and invited eight major fluoropolymer and telomer manufacturers to commit to eliminating precursor chemicals that can break down into PFOA; in one case, DuPont has since introduced so-called short-chain chemistry, whereby the length of perfluorinated chains within polymers are kept below a threshold in order to avoid degradation into PFOA. In other applications, usage of fluoropolymers in products that come in sustained contact with the human body or in disposable items intended for landfilling after consumption must be minimized.